Safety device for machine tools



Feb. 1, 1944. M|KAM| 2,340,547

SAFETY DEVICE FOR MACHINE TO0Ls Filed Aug. 9, 1940 Fig. 1

E f i I! I r- AMPLIFIER 7 Haze 20 pram-75 Fig.

3 Fig. 4

L A L B Bnvento;

Patented Feb. 1, 1944 2,340,547 I SAFETY DEVICE FOR MACHINE TOOLSShlnkuro Mikami, Tokyo, Japan; vested in the Alien Property CustodianApplication August 9, 1940, Serial No. 352,035

2 Claims.

safety device .for machine tools which is simple in construction andpositive in operation for stopping the feed or a work at once when thereoccurs an accident to cause objections to the operation of the machinetool owing to some failure such as slowing down of the cutting tool andthe like causes.

In machine tools such as milling machines an article to be machined,that is, work, is fed to the cutting tool while the latter is rotatingand if some teeth of, the cuttingtool are broken the revolving toolshould be stopped at once. Here-- tofore for this purpose a frictionclutch or slipping belt has been used in some driving part of themachine tool in order to protect the cutting tool by allowing thedriving mechanism to slip. But if in such a case the work is fed to thecutting tool either the work or the cutting tool might be damaged. Inorder to avoid such danger the work should be stopped as soon as thecutting tool is stopped. I

This invention is tosatisfy the above requirements by means ofphoto-electric current usin alternating current amplifiers, filters,r'ectifiers and relays to control the work feeding motor.

In the drawing, Fig. 1 is a diagrammatic elevation of a plane-miller towhich is applied my invention; Fig. 2 is a diagrammatic view showing anarrangement of my invention and Figs. 3 and 4 are characteristic curvesof the current taken for the explanation otoperation of my invention.

Referring to the accompanying drawing, I represents a cutting tool suchas a milling cutter and 2 is an article to be worked or machined. 3 is aworking table which is moved to-and-iro by means of a half nut 4 securedthereto and a screw shaft 5 which latter is driven by a work feedingmotor 8 through gear wheels I and 6. 9 represents an electric motor fordriving the cutting tool I through the gear wheels I0, triction clutchII and a train of gear, wheels I! and I3. The friction clutch II isdesigned to slip automatically when the resistance of the cutting tool Iincreases beyond a predetermined value so that the tool may be safelyprotected, but it is very dangerous it the work 2 is not stoppedimmediately in such a case.

In accordance .with this invention, a light source I4 is arranged in acertain relative position to the cutting tool I itself to project thelight beam through a lens I5 and the clearance space between teeth ofthe cutter I upon a photo-electric tube I8 as shown in Fi 2. Thephoto-electric tube is connected to the amplifying unit I! consisting ofan alternating current amplifIer filter and rectifier which is excitedfrom an exciting source through the lines I8, a filter and a rectifier.The output terminals of the amplifying 'unit I! are connected to a relayI9, the contact 20 of which controls the circuit of an electromagneticdevice 2I operating the main switch of the work feeding motor 8. In thiscase the teeth of the cutting tool themselves are taken advantage of asa light interrupter for passing or; interrupting the light beamprojected from the light source. As a modified embodiment of myinvention, instead of using the cutting tool itself as a lightinterrupter a slotted disc may be arranged to rotate at a speedproportional to that of the cutting tool and to pass and interrupt thelight beam projected to the photo-electric' tube continuously.

In the above described device of this invention,- the photo-electrictube It generates pulsating current as shown by the curve A of Fig. 3during the normal revolution of the cutting tool, so that thesubstantial amount of'alternating' current is amplified, filtered, andrectified in the amplifying unit I! and the rectified current issupplied to the. relay to hold the power switch of the motor 8 to theclosed condition. If some teeth of the cutting tool I are caught causinga greater resistance to allow the clutch II to slip the cutting tool Iis slowed down or stopped. Then the voltage induced in thephoto-electric tube I6 will become a pulsating voltage of very lowfrequencies as shown by the wave formof the curve B of Fig. 4 so thatthe current may be impeded by means of a high pathfllter in theamplifying unit II. Accordingly the. relay I9 is practically deenergizedand interrupts the current supplied to the motor 8 and the'sliding tablecarrying the work is stopped.

Thus thepresent invention has characteristics of safely protecting themachine tool by positively stopping the feed of work to the cutting toolwhen the latter is stopped or is under abnormal operative conditions.

between teeth of the cutting tool on said photoelectric tube to generatepulsating current in the output circuit of said tube and a rectifyingunit connected with said photo-electric tube to transmit the pulsatingcurrent to the circuit controlling device 0! an electric motor whichfeeds the work to said cutting tool.

2. A'saiety device for machine tools comprising a light source arrangedin a suitable relation to the cutting tool secured to the machinetool torotate, a photo-electric tube, an optical means to

